1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, etc., and more particularly, to an image forming apparatus including a film-heating-type fixing unit including a heater and a film that moves while sliding on the heater.
2. Description of the Related Art
When an image forming apparatus designed for use in an area where a commercial power supply of a voltage of 100 volts (in nominal value, with an actual value in a range of, for example 100 to 127 volts (hereinafter, such a power supply will be referred to simply as a 100-volt power supply)) is supplied, is used in an area where a commercial power supply of a voltage of 200 volts (in nominal value, with an actual value in a range of, for example 200 to 240 volts (hereinafter, such a power supply will be referred to simply as a 200-volt power supply)), maximum electric power available for a heater in a fixing unit becomes 4 times higher. An increase in the maximum electric power available for the heater can cause a significant increase in a harmonic current or flicker generated during a process of controlling power of the heater by means of a phase control, a wavenumber control, etc. Besides, if thermal runaway occurs in the fixing unit, electric power associated with the thermal runaway is 4 times greater, and thus circuits used need to be capable of quickly responding. Therefore, the most common way to allow a single image forming apparatus to be used in both 100-volt and 200-volt power supply areas is to select a heater with a proper resistance depending on the area and install the selected heater.
A technique has been proposed to realize an apparatus for universal use in both 100-volt and 200-volt commercial power supply areas by switching the resistance of the heater using a relay or other switching devices. More specifically, for example, Japanese Patent Laid-Open No. 7-199702 discloses an apparatus in which first and second resistance heating elements are formed on a heater substrate, and the apparatus is adapted to be capable of switching between a first operation mode in which the first and second resistance heating elements are connected in series and a second operation mode in which the first and second resistance heating elements are connected in parallel whereby it is possible to switch the resistance of the heater depending on the commercial power supply voltage such that the apparatus can be used regardless of where the commercial power supply voltage is 100 volts or 200 volts.
In the technique in which the first and second resistance heating elements are connected in series or in parallel depending on the commercial power supply voltage, it is possible to switch the resistance of the heater without changing the heating area of the heater. In other words, the two resistance heating elements generate heat regardless of whether the apparatus is used in the 100-volt area or 200-volt area, and thus a fixing nip has a constant temperature distribution in a recording sheet conveying direction regardless of the area in which the apparatus is used. As a result, the performance of fixing toner images does not depend on the area in which the apparatus is used.
However, the heat distribution in a lateral direction of a heater can become different between a state in which the two resistance heating elements are connected in series and a state in which the two resistance heating elements are connected in parallel, and this difference can cause a difference in quality of a fixed toner image. An investigation has been performed to find a cause thereof, and it turns out that a temperature distribution in a direction of a film rotation inside the fixing nip can be different between the series connection and the parallel connection, and this different in temperature distribution can cause the above problem. In the fixing apparatus of the film heating type, heat generated by the heater is transferred by the rotating film to a downstream part, and thus the temperature tends to become higher in a downstream part in the fixing nip than in an upstream part during a rotating operation. In general, resistance heating elements have a non-zero TCR (Temperature Coefficient of Resistance). Therefore, the resistance thereof changes with temperature. If a difference occurs in resistance between the two resistance heating elements, there can be a difference in current flowing in each resistance heating element between the series connection and the parallel connection, which can bring about a difference in heat distribution. As a result, a difference occurs in the amount of heat given to a recording sheet passed through the nip between the series connection and the parallel connection, which can create a difference in image quality related to a fixing performance or the like. The difference can be significant in particular when the resistance heating element has a large TCR (Temperature Coefficient of Resistance).